如何在mysql语句中做IF语句作为SELECT语句的一部分

Question

I'm comparing search engine rankings for URLs and I have 3 columns: URL, Rank, Previous rank. What I'd like to do is add a fourth column which says whether the rank has gone up or down.

For example

URL               | Rank | Previous_Rank
example.com/page1 |  2 | 16
example.com/page2 |  2 | 11
example.com/page3 |  1 | 14
example.com/page4 |  1 | 4
example.com/page5 | 101| 7
example.com/page6 | 101| 14
example.com/page7 | 101| 7
example.com/page8 |  6 | 17
example.com/page9 |  10| 17
example.com/page10|  19| 1

I'd like another column to return:

 URL                  | Rank | Previous_Rank | Movement
    example.com/page1 |  2 | 16  | Up
    example.com/page2 |  2 | 11  | Up
    example.com/page3 |  1 | 14  | Up
    example.com/page4 |  1 | 4   | Up
    example.com/page5 | 101| 7   | Down
    example.com/page6 | 101| 14  | Down
    example.com/page7 | 101| 7   | Down
    example.com/page8 |  6 | 17  | Up
    example.com/page9 |  10| 17  | Up
    example.com/page10|  19| 1   |Down

I'm using HeidiSQL which is mysql. The data here is part of a larger table and is pulled together with this SELECT statement:

select 
    URL,
    Rank,
    Previous_Rank
from URL_Changes
where
    date = "2017-06-14"
group by URL
order by 2
;

So my question is, how do I edit that select statement to bring back that extra column?

Thanks.

Answer 1

You use a CASE statement for this:

select 
    URL,
    Rank,
    Previous_Rank,
    CASE WHEN Rank < Previous_Rank THEN 'Up' WHEN Rank > Previous_Rank THEN 'Down' WHEN Rank = Previous_Rank THEN 'No Change' END AS Movement
from URL_Changes
where
    date = "2017-06-14"
group by URL
order by 2
;

In MySQL there is also an IF() function, but CASE is used on nearly every RDBMS and IF() requires nesting to test for multiple scenarios, so Case is generally the better choice.